A bandgap voltage reference is a voltage reference used in integrated circuits (ICs) for producing a fixed or constant voltage independent of power supply variations, temperature changes and loading. A bandgap voltage is the combination of a bipolar (or diode) base-emitter junction voltage (Vbe) and a PTAT (proportional to absolute temperature) voltage. Vbe is roughly 650 mV at room temperature and has a negative temperature coefficient (TC). The PTAT voltage has a positive TC which, when added to the negative TC of the Vbe, creates a low-temperature coefficient reference of about 1.24 volts. That is to say that the reference varies very little over temperature.
In conventional bandgap voltage reference designs, the ΔVbe (PTAT voltage) is the difference of two diode voltages biased at different current densities. For example, the PTAT voltage may be the difference between two diodes biased at the same current level where the second diode is sized 8 times larger than the first diode for an 8:1 current density difference. This results in a PTAT voltage of Vt*ln(8) or about 54 mV at room temperature. Alternatively the same voltage could be generated by using two equal size diodes with the first diode biased at 8 times the bias current of the second diode.
Pressure from the package (e.g., a plastic package) can introduce a piezoelectric effect on the integrated circuit die that can shift Vbe and PTAT voltage (ΔVbe). The effect on the bandgap voltage due to the shift in Vbe is 1:1. For example, a 1 mV shift in Vbe shifts the bandgap voltage by 1 mV. However, the gain of the PTAT voltage is increased by a factor in the range of about 5-20 (e.g., 10) in the bandgap. Thus, most of the package shift is due to PTAT voltage sensitivity.